Formation of loops or coils of a material passing through an opening of an object presents certain difficulties of manufacture. Constructing cores for electromagnetic induction devices such as transformers presents a typical manufacturing situation. One manufacturing method involves assembling individual core sections or segments to form a core that fills the opening and partially or wholly surrounds one or more transformer inductance coils. Another method involves winding a ribbon of material such as steel about a mandrel to pre-form the core in the desired shape, severing the core, and reassembling it to fill and surround the inductance coil.
Transformer cores constructed by winding a ribbon of transformer steel into a shape of a ring or a squared-off "O" offer certain advantages over transformer core assembled from individual laminar sections of transformer steel: "wound[ cores pack steel very tightly, hence permitting construction of transformers that are compact and that exhibit comparatively low electrical losses. Wound cores, moreover, allow rapid assembly of transformers and permit construction of transformers that are comparatively quiet.
However, wound cores have in the past had certain disadvantages. Compared with conventional laminar cores, pre-wound transformer cores are expensive as they require extensive manufacturing operations prior to their being available for transformer manufacturing. After winding about a mandrel is completed, the cores are annealed for many hours at high temperatures, often in a reduction atmosphere of hydrogen or nitrogen. The annealing process relaxes the strains introduced into the metal by the winding process.
Subsequent to annealing, the cores are subjected to a pressurized varnishing process and are then baked at elevated temperatures to cure the varnish. After baking, the cores typically are cut in a direction diametric to the direction of the wound steel and in a way that yields two pieces that eventually fit together precisely. After this cutting operation, the cut surfaces are often etched with acid to remove from the cut surface small metal burrs that result from the cutting action. Such burrs are undesirable because they tend to bridge the cut pieces of steel and, in service, tend to cause undesirable, heat-producing eddy-currents.
After etching of the cut surfaces, the pre-wound core pieces customarily are lapped at the cut surfaces, then are numbered for later matching of halves. They are then dipped in a plastic substance to protect the cut and etched surfaces from scratches and marring and to keep matching core pieces from being separated. When the cores are used in the manufacture of transformers, the two matching pieces are temporarily separated, and one or more inductance coils are arranged together with one or more wound cores to form a magnetic/electrical circuit such as is commonly used in transformers, inductors or saturable reactors. The previously cut core pieces are then customarily kept securely joined by means of a strapping band made of steel, stainless steel, or some other material exhibiting high tensile strength.
Other methods avoid previously winding the core by transferring core ribbon from a roll to form a coil larger in diameter yet thinner in thickness which, driven by friction rollers, revolves freely through the opening of the inductance coil. In some methods the coil is then simply increased in thickness to approach the outer bounds of the inductance coil opening. Other methods proceed, after the original roll is emptied, to continue revolving the coil in the same direction about a leg of the inductance coil after attaching a terminal end of the ribbon to the inductance coil. The ribbon thereby is wound directly about the inductance coil itself. In yet another method, the newly-formed coil is tightened by pulling the ribbon toward the original roll after attaching the terminal end to the inductance coil.